Dynamic channel equalizers and wavelength selective switches are considered necessary and highly cost-effective building blocks for implementing dynamic wavelength channel equalization, blocking, and switching functions in next generation long haul and metro networks.
A favorite device architecture used in these designs is one that demultiplexes the spectral content of the input light signal into an array of wavelength channels using a free-space diffraction grating. Once separated, each wavelength channel can be modulated or redirected along a different optical path by an intermediate array of control elements. After being manipulated, each of the wavelength channels can be multiplexed back together into one or several output ports by folding the path of the light back upon the same grating or directing the path of light onto another grating of equal dispersion.
When a single fiber optic input and output port is used, the device functions as a dynamic channel equalizer or wavelength blocker. When the light from individual wavelength channels can be redirected toward any of N output ports the device functions as a 1×N wavelength selective switch.
One means known in the prior art for the intermediate manipulation of wavelength channels is to use an array of liquid crystal modulators. Such modulators usually function by changing the polarization state of each wavelength channel and can be used to form a wavelength blocker or a 1×2 wavelength selective switch. While it is possible to extend this approach to higher port counts, this approach is very costly and complicated.
A simpler and more extendible approach to higher port count wavelength routing is to use an array of micromachined reflective mirrors for individual wavelength channel manipulation. Some approaches for achieving this include a mirror array which routes light in each wavelength channel by either changing the angle of the return optical beam or by displacing the path of the return optical beam using mirrors in a paired plane configuration or a paired retro-reflector configuration.
While such wavelength selective switching is extensively mentioned in the prior art, a design for implementing both switching as well as equalization has proven to be neither a simple nor an obvious extension of that art, from a control and design standpoint. In addition to these considerations, the prior art has failed to address adequately other performance criteria including high channel bandwidth with flat-top channel performance, low polarization independence loss, low vibration sensitivity and therefore high resonant frequency steering elements, high extinction ratios greater than 40 dB, and very low levels of electrical and optical channel cross-talk.
Furthermore, some DWDM applications require wavelength selective switching with seamless optical performance in the spectral region in between adjacent wavelength channels with frequency spacing typically set at 50 GHz or 100 GHz on the ITU grid. Whereas present liquid crystal modulator arrays offer seamless spectral performance in the non-blocking state, micromachined arrays of actuators typically have dips in insertion loss in between the actuator channels. In these regions chromatic dispersion and polarization depended loss (PDL) are often uncontrolled. The existence of the dips also reduces the effective usable bandwidth associated with each actuator and impose the limitation that each actuator must have a one-to-one correspondence to each wavelength channel. This can limit the flexibility of such a device when larger or smaller spectral spacing is required. While a wavelength blocker based upon diffractive micromachined ribbon elements does offer seamless performance across the telecom spectral band, it is difficult to build a wavelength switching device with higher port counts (N>1) based upon this technology, as it is difficult to collect light from the higher order diffracted modes and redirected in an efficient manner into additional ports with low insertion loss.
Thus a new type of actuator array is needed which can efficiently redirect spectrally separated light towards multiple output ports with near spectrally seamless performance.